Question

The pH of which salt is independent of its concentration:
(i)$(C{{H}_{3}}COO){{C}_{5}}{{H}_{5}}NH$ (ii)$Na{{H}_{2}}P{{O}_{4}}$ (iii)$N{{a}_{2}}HP{{O}_{4}}$ (iv)$N{{H}_{4}}CN$
(A) 1, 2, 3, 4
(B) 1, 4
(C) 2, 3
(D) 1, 2, 3

Answer 1

Hint: pH of any salt is its hydrogen potential or capacity to liberate hydrogen ions. Salts are made up of neutralization between acid and bases. The pH of a salt does not depend on concentration when a salt is amphiprotic in nature (donate and accept protons) or when it is made up of a weak acid and a weak base.

Complete answer:
pH of any salt is the power or potency of hydrogen ions, also called the hydrogen ion concentration. We have been given salts, and asked that the pH of which salt is independent of its concentration.
The salts that are amphiprotic that is they can donate as well as accept a proton in the solution, their concentration does not affect the pH as they are accepting and donating the hydrogen ion. The concentration of salt that is formed by a weak acid and a weak base also does not affect the pH as they have very less dissociation to liberate hydrogen ions.
Among the given salts,
$(C{{H}_{3}}COO){{C}_{5}}{{H}_{5}}NH$ is a salt of a weak acid $C{{H}_{3}}COOH$ and weak base${{C}_{5}}{{H}_{5}}N{{H}_{2}}$
$Na{{H}_{2}}P{{O}_{4}}$is an amphiprotic salt as it accepts and donates proton as:
$\begin{align}
  & {{H}_{2}}P{{O}_{4}}^{-}+{{H}^{+}}\to {{H}_{3}}P{{O}_{3}} \\
 & {{H}_{2}}P{{O}_{3}}^{-}\to HP{{O}_{4}}^{2-}+{{H}^{+}} \\
\end{align}$
$N{{a}_{2}}HP{{O}_{4}}$ is also an amphiprotic salt as it accepts and donates proton as:
$\begin{align}
  & HP{{O}_{4}}^{-}+{{H}^{+}}\to {{H}_{2}}P{{O}_{4}}^{-} \\
 & HP{{O}_{4}}^{-}\to P{{O}_{4}}^{2-}+{{H}^{+}} \\
\end{align}$
$N{{H}_{4}}CN$ salt is a salt of weak acid $HCN$ and weak base $N{{H}_{4}}OH$
Hence, the pH of all the given salts is independent of its concentration.

So option A is correct.

Note:
Salts are formed by the neutralization of acids and bases. The salt of weak acid and strong base are basic salts, while the salts of strong acids and weak bases are acidic salts. Salts of strong acids and bases are neutral salts. Thus the pH depends on the liberation of hydrogen ions in various salts.